Conventional data recording disk files utilize one or more rotatable disks with concentric data tracks, one or more heads for reading or writing data onto the various tracks, and an actuator connected by a support arm assembly to the heads for moving the heads to the desired tracks and maintaining them over the track centerlines during read or write operations. The disks are mounted in a stack on a hub which is attached to a rotatable shaft. The disk drive motor for rotating the disks may be a DC brushless motor which is located in the region between the hub and the shaft. The use of this type of "in-hub" motor results in a complete disk stack subassembly, which includes the drive motor, shaft, hub, bearing assemblies and disk stack, and which thereby eliminates the need for a drive motor located externally of the disk file housing.
Disk files which use in-hub disk drive motors are described in U.S. Pat. Nos. 4,005,490; 4,519,010 and 4,607,182. While the disk files described in these patents use either fixed heads or a rotary (i.e., "swinging arm") actuator, it is also possible to use a linear actuator in disk files which have in-hub motors. One common form of construction of the housing for such disk files is for both the actuator and the disk stack subassembly to be mounted on a rigid base plate or dish. A cover plate is then placed over the base plate or dish to enclose the actuator and to support the other end of the disk stack subassembly. Typically, a surface of the middle disk in the stack contains head positioning servo information, and the corresponding head is a dedicated servo head which reads the servo information to position the data heads on the data tracks. One problem with this type of construction is that bending of the base plate, caused by unsymmetrical thermal expansion of the base plate and cover plate and vibration of the disk stack relative to the actuator caused by movement of the actuator, can result in head-to-track misregistration or head-to-head misregistration.
Assignee's U.S. Pat. No. 4,743,995 describes a disk file with an in-hub motor and two linear actuators. The disk stack subassembly in the '995 patent includes a stationary shaft which is mounted to cut-outs on spaced-apart arms of a central box frame. Separate housings for the linear actuators are bolted to the box frame at equal angles to the plane of symmetry of the box frame.